A liquid crystal display device is a flat display device having excellent characteristics such as high definition, thin form, light weight and low consumption of electricity, and recently the market size of it is rapidly expanding due to the increase in display ability, the increase in producing ability, and the increase in competitive power of the price against other display devices.
For a liquid crystal display device in twisted nematic mode (TN mode) that has been general so far, an orientation process is carried out, in which a long axis of a liquid crystal molecule with positive permittivity anisotropy is oriented substantially in parallel to the surface of substrates, and the long axis of a crystal liquid molecule is twisted approximately 90° between the above and below substrates in a thickness direction of a liquid crystal layer. Applying a voltage on this liquid crystal layer allows the liquid crystal molecule to stand in parallel to an electric field and twisted orientation is eliminated. The liquid crystal display device in TN mode uses the change of optical rotation accompanying the change of orientation of the liquid crystal molecule due to a voltage, so as to control transmitted light volume.
The liquid crystal display device in TN mode has wide production margin and excellent productivity, but on the other hand has a problem in display ability, particularly in viewing angle characteristics. To put it concretely, there was a problem that when the display face of the liquid crystal display device in TN mode is observed from the side, the contrast ratio of display greatly lowers, and when the image in which a plurality of gradations from black to white are clearly observed from the front is observed from the side, the difference in luminance between gradations becomes very unclear. Further, a phenomenon in which gradation characteristics of display are inverted and the darker part in front view observation is seen brighter in side view observation (so-called gradation inversion phenomenon) is also problematic.
Recently, as liquid crystal display devices that improve viewing angle characteristics in the liquid crystal display devices in TN mode, such modes have been developed as in-plane switching mode (IPS mode), multi-domain vertical aligned mode (MVA mode) and axially symmetric aligned micro-cell mode (ASM mode).
Each of the liquid crystal devices in these new modes (wide viewing angle mode) solves the above concrete problems as to viewing angle characteristics. Namely, the problem that the contrast ratio of display greatly decreases or display gradation inverses when the display face is observed from the side is never generated.
However, under the condition where the improvement in display quality of a liquid crystal display device advances, as a problem of viewing angle characteristics, a new problem that γ characteristics in front view observation and γ characteristics in side view observation are different, namely, a new problem of viewing angle dependency of γ characteristics has appeared. Here, γ characteristics are gradation dependency of display luminance, and a difference in γ characteristics between when viewed from the front and when viewed from the side means that the state of gradation display is different according to the direction of observation, and therefore it is particularly problematic in displaying images such as photographs and in displaying TV broadcasting.
The problem of viewing angle dependency of γ characteristics is more prominent in MVA mode or ASM mode than in IPS mode. On the other hand, IPS mode has a difficulty in producing with good productivity panels with a high contrast ratio in front view observation, compared with MVA mode or ASM mode. In terms of these points, it is desirable to improve viewing angle dependency of γ characteristics in the liquid crystal display device particularly in MVA mode or ASM mode.
The inventor of the present application proposes a multi-picture element driving method as a method for improving the above viewing angle dependency of γ characteristics, in Japanese Laid-Open Patent Application No. 2004/62146 (Tokukai 2004-62146) (published date; Feb. 26, 2004, corresponding US application; US2003/0227429A1). First, this multi-picture element driving method is explained with reference to FIGS. 5 through 7.
The multi-picture element driving is a technology for composing one display picture element by using two or more sub picture elements having different luminance levels, so as to improve viewing angle characteristics (viewing angle dependency of γ characteristics). First, the principle of this technology is briefly explained.
FIG. 5 illustrates γ characteristics of a liquid crystal display panel (gradation (voltage)-luminance ratio). The full line in FIG. 5 shows γ characteristics in front view observation in a general driving method (in which one display picture element is not composed of a plurality of sub picture elements), and in this case, the most normal visibility can be gained. Further, the broken line in FIG. 5 shows γ characteristics in side view observation (viewing from the side) in a general driving method, and in this case, a shift occurs to normal vision (namely, vision in front view observation) and the amount of a shift is small in a place showing high luminance and low luminance, and large in a place showing halftones.
In the case of obtaining targeted luminance in one display picture element, the multi-picture element driving method performs display control so that in a plurality of sub picture elements having different luminance levels, the average luminance among them becomes targeted luminance. And in the multi-picture element driving method, γ characteristics in front view observation is set so as to obtain the most normal visibility, as with the case of the general driving method (the same characteristics as γ characteristics of a full line in FIG. 5). On the other hand, as for visibility from the side in the multi-picture element driving method, for example, in order to obtain targeted luminance in a halftone where uneven luminance usually increases, the multi-picture element driving method causes the sub picture elements to have the regions around high luminance and low luminance where uneven luminance decreases, so that the picture element as a whole can obtain the targeted luminance in a halftone by balancing luminance levels of those sub picture elements. This decreases uneven luminance, and γ characteristics shown by a chain line in FIG. 5 can be obtained.
Next, one example of a structure of a liquid crystal display device for performing multi-picture element driving is illustrated in FIG. 6. As illustrated in FIG. 6, a picture element 10 corresponding to one display picture element is composed of sub picture elements 10a and 10b respectively including sub picture element electrodes 18a and 18b, and TFTs (Thin Film Transistor) 16a and 16b, and subsidiary capacities (CS) 22a and 22b are respectively connected to the sub picture elements 10a and 10b. Note that FIG. 6 illustrates one example of the structure of a picture element when one picture element is composed of two sub picture elements, to put it concretely, the structure in which the areas of the sub picture elements are substantially the same as each other and the sub picture elements are placed in a longitudinal direction, but the effect of the present invention is not limited to the arrangement illustrated in FIG. 6. As for the areas of each sub picture element, they may be different from each other as well as substantially the same as each other illustrated in FIG. 6. Concretely, it is possible to make the area of a sub picture element with high luminance in a halftone display condition smaller than the area of a sub picture element with low luminance, or on the contrary to make the area of a sub picture element with high luminance larger than the area of a sub picture element with low luminance. In terms of the improvement in viewing angle characteristics, the former is preferable. Further, as for the disposition of sub picture elements, instead of disposing above and below the sub picture elements with different luminance levels in displaying halftones, it may be that the lateral direction of the row of picture elements is made a standard axis, and the sub picture elements are disposed along the axis. In this case, the distribution of display polarity of the sub picture elements becomes like dot inversion, and therefore it is preferable in terms of display quality. FIGS. 10 (a) and (b) illustrate examples of disposition of sub picture elements placed over a plurality of picture elements. ∘ in FIGS. 10 (a) and (b) show sub picture elements with high display luminance, and + and − in ∘ show electric polarity of picture elements (when the potential of a picture element electrode (sub picture element electrode) is high relative to the potential of a counter electrode, it is +, and when low, it is −).
FIG. 10(a) illustrates a case according to the disposition in FIG. 6, and FIG. 10(b) illustrates a case according to the above preferable disposition. In FIG. 10(a), the sub picture elements with high luminance in a halftone display condition are disposed in a checkered pattern (the weighted center of luminance of a picture element does not correspond to that of luminance of a sub picture element with high luminance, but they are disposed in a condition of high dispersibility on a screen), and noting either + or − of display polarity out of sub picture elements with high luminance shows that they are disposed in a line in the direction of a row. Namely, the disposition of the sub picture elements with high luminance is like line inversion. On the other hand, in FIG. 10 (b), a sub picture element with high luminance is disposed in the center of a picture element (the weighted center of luminance of a picture element corresponds to that of luminance of a sub picture element with high luminance), and the display polarity of a sub picture element with high luminance shows the form of dot inversion as with the display polarity of a picture element. According to these conditions, FIG. 10 (b) is preferable to FIG. 10 (a) in terms of the disposition of a sub picture element.
Further, the shape of a sub picture element is not limited to a rectangle. Particularly, in the case of MVA mode, the shape may be a structure of dividing along rib or slit, namely, a structure such as a triangle or a rhomboid, and such a shape is preferable in terms of an open area ratio of a panel (see FIG. 10 (c)).
Gate electrodes of the TFTs 16a and 16b are connected to a common (same) scan line 12, and a source electrode is connected to a common (same) signal line 14. The subsidiary capacities 22a and 22b are respectively connected to subsidiary capacity wires (CS bus lines) 24a and 24b. 
The subsidiary capacities 22a and 22b are respectively composed of subsidiary capacity electrodes electrically connected to the sub picture element electrodes 18a and 18b, subsidiary capacity counter electrodes electrically connected to the subsidiary capacity wires 24a and 24b, and insulating layers (not shown in figures) disposed between these electrodes. The subsidiary capacity counter electrodes of the subsidiary capacities 22a and 22b are independent of each other, and have a structure for being supplied with subsidiary capacity counter voltages from the subsidiary capacity wires 24a and 24b, the subsidiary capacity counter voltages being different from each other.
Further, the driving signals of the liquid crystal display device illustrated in FIG. 6 are illustrated in FIGS. 7(a) through 7(f). FIG. 7(a) shows voltage waveform Vs of the signal line 14, FIG. 7(b) shows voltage waveform Vcsa of the subsidiary capacity wire 24a, FIG. 7(c) shows voltage waveform Vcsb of the subsidiary capacity wire 24b, FIG. 7(d) shows voltage waveform Vg of the scan line 12, FIG. 7(e) shows voltage waveform Vlca of the sub picture element electrode 18a, and FIG. 7(f) shows voltage waveform Vlcb of the sub picture element electrode 18b. Further, broken lines in FIGS. 7(a) through 7(f) show voltage waveform COMMON (Vcom) of a counter electrode (not shown in FIG. 6).
First, in time T1, the voltage Vg changing from VgL to VgH allows the TFT16a and the TFT16b to be conduction states (ON-states) simultaneously, and thereby the voltage Vs of the signal line 14 is transmitted to the sub picture element electrodes 18a and 18b, with a result that the sub picture elements 10a and 10b are charged. In the same way, the subsidiary capacities 22a and 22b of the respective sub picture elements are charged by the signal line 14.
Next, in time T2, the voltage Vg of the scan line 12 changing from VgH to VgL allows the TFT16a and the TFT16b to be non-conduction states (OFF-states) simultaneously, and thereby the charge of the sub picture elements 10a and 10b and the subsidiary capacities 22a and 22b is finished, with a result that the sub picture elements 10a and 10b and the subsidiary capacities 22a and 22b are electrically insulated from the signal line 14. Note that immediately after that, due to drawing phenomenon caused by the effect of parasitic capacitance or the like included by the TFT16a and the TFT16b, the voltage Vlca of the sub picture element electrode 18a and the voltage Vlcb of the sub picture element electrode 18b decrease by substantially the same voltage Vd, and they become:Vlca=Vs−Vd; andVlcb=Vs−Vd. 
Further, at the time, the voltage Vcsa of the subsidiary capacity wire 24a and the voltage Vcsb of the subsidiary capacity wire 24b are:Vcsa=Vcom−Vad; andVcsb=Vcom+Vad. 
In time T3, the voltage Vcsa of the subsidiary capacity wire 24a connected to the subsidiary capacity 22a changes from Vcom−Vad to Vcom+Vad, and the voltage Vcsb of the subsidiary capacity wire 24b connected to the subsidiary capacity 22b changes from Vcom+Vad to Vcom−Vad. Along with this change of voltages of the subsidiary capacity wires 24a and 24b, the voltages Vlca and Vlcb of each sub picture element electrode change as follows:Vlca=Vs−Vd+2×K×Vad; andVlcb=Vs−Vd−2×K×Vad. 
Note that K=CCS/(CLC(V)+CCS). Here, CLC(V) is the value of capacitance of liquid crystal capacity in the sub picture elements 10a and 10b, and the value of CLC(V) depends on effective voltage (V) applied to liquid crystal layers of the sub picture elements 10a and 10b. Further, CCS is the value of capacitance of the subsidiary capacities 22a and 22b. 
In time T4, Vcsa changes from Vcom+Vad to Vcom−Vad, and Vcsb changes from Vcom−Vad to Vcom+Vad, and Vlca and Vlcb also change fromVlca=Vs−Vd+2×K×Vad Vlcb=Vs−Vd−2×K×Vad toVlca=Vs−Vd Vlcb=Vs−Vd. 
In time T5, Vcsa changes from Vcom−Vad to Vcom+Vad and Vcsb changes from Vcom+Vad to Vcom−Vad by twofold Vad, and Vlca and Vlcb also change fromVlca=Vs−Vd Vlcb=Vs−Vd toVlca=Vs−Vd+2×K×Vad Vlcb=Vs−Vd−2×K×Vad. 
Vcsa, Vcsb, Vlca and Vlcb repeat alternately the change in the T3 and T5. The interval or phase of repetition of the T3 and T5 should be suitably set in consideration of a driving method of a liquid crystal display device (a method such as a polarity inversion method) and of a display state (such as flicker or rough surface of display) (for example, as for the interval of repetition of the T3 and T5, 0.5 H, 1H, 2 H, 4 H, 6 H, 8 H, 10 H, 12 H, . . . can be set (1 H is 1 horizontal scan period)). This repetition is continued until the next time the picture element 10 is rewritten, namely, until the time being equivalent to T1. Therefore, the effective values of the voltages Vlca and Vlcb of the sub picture element electrodes are:Vlca=Vs−Vd+K×Vad;andVlcb=Vs−Vd−K×Vad. 
Therefore, effective voltages V1 and V2 applied to liquid crystal layers of the sub picture elements 10a and 10b are:V1=Vlca−Vcom;andV2=Vlcb−Vcom. Namely,V1=Vs−Vd+K×Vad−Vcom;andV2=Vs−Vd−K×Vad−Vcom. 
Therefore, the difference of effective voltages applied to liquid crystal layers of the respective sub picture elements 10a and 10b, ΔV12 (=V1−V2), becomes ΔV12=2×K×Vad, and it is possible to apply to the sub picture elements 10a and 10b voltages different from each other.
However, according to the above conventional structure, there is a problem that uneven luminance appearing in a lateral streak occurs when a certain gradation (halftone) is displayed all over the display screen of a liquid crystal display device with large size and high definition. The cause of the occurrence of the uneven luminance appearing in a lateral streak is explained below with reference to FIGS. 8 and 9.
FIG. 8 is a plane view illustrating a relation of disposition between activation drivers and subsidiary capacity wires.
In a liquid crystal display device with large size and high definition, as illustrated in FIG. 8, it is general to use a plurality of gate drivers 30 and source drivers 32 for activating the scan line 12 (see FIG. 6) and the signal line 14 (see FIG. 6) in a display region. Note that in FIG. 8, the scan line 12 and the signal line 14 are not shown.
Further, all the subsidiary capacity wires 24a are connected to a subsidiary capacity main line 34a, and the voltage Vcsa is inputted to the subsidiary capacity main line 34a through several input points. In general, the input points of the voltage Vcsa are set between gate drivers 30 that are separately disposed. Note that FIG. 8 illustrates a structure for applying the subsidiary capacity voltage Vcsa to the subsidiary capacity wire 24a, and the subsidiary capacity voltage Vcsb is applied to the subsidiary capacity wire 24b with the same structure.
Here, according to the structure illustrated in FIG. 8, in the subsidiary capacity wire 24a (such as point B) being far from the input point of the voltage Vcsa (such as point S), compared to the subsidiary capacity wire 24a (such as point A) being near to the input point of the voltage Vcsa, the effect of electric charge due to electric resistance and parasitic capacitance included by the subsidiary capacity main line becomes large, so that voltage waveforms are blunted greatly, as illustrated in FIG. 9. Note that in FIG. 9, a full line shows a waveform of a voltage, supplied to the input point (point S), for driving the subsidiary capacity wire, a broken line shows the voltage waveform of the subsidiary capacity wire 24a (point A) near to the input point, and chain line shows the voltage waveform of the subsidiary capacity wire 24a (point B) far from the input point.
When the voltage waveforms of each subsidiary capacity wire 24a are different according to the distance from the input point, the potentials of each subsidiary capacity wire 24a vary depending upon timing when the gate of TFT is turned OFF. This becomes the cause of the occurrence of uneven luminance appearing in a lateral streak. The reason is explained below.
According to the above explanation by use of FIG. 7, voltages applied to liquid crystal layers in the multi-picture element driving are influenced by the voltages Vcsa or Vcsb of the subsidiary capacity wires, as well as by the voltage Vs of the signal line. The concrete performance of Vcsa or Vcsb is as follows.
In a general liquid crystal display device, liquid crystal capacity of each picture element is charged with a voltage from the signal line through its TFT element, after of which, it maintains the value of this signal voltage until next charging starts. On the contrary, in the liquid crystal display device of the multi-picture element driving, after charging is finished (after a TFT element is turned OFF), i.e. after time T2 of FIG. 7, the voltage oscillation of the CS bus line (Vcsa or Vcsb) oscillates the voltage of the liquid crystal capacity through the subsidiary capacity. Thus, the voltage of the liquid crystal capacity is influenced by the voltage oscillation of the CS bus line. What matters here is that, voltage oscillation of the liquid crystal capacity accompanying voltage oscillation of the CS bus line refers to the voltage of the CS bus line at the time when TFT element is turned OFF, i.e. at the time T2 of FIG. 7. That is, the voltage of the CS bus line increasing and decreasing (oscillating) from this reference voltage is superposed on the voltage of liquid crystal capacity at the time T2 (in a narrow sense, a voltage obtained by subtracting Vd from a charge voltage of the signal line). In other words, the influence of the voltage oscillation of the CS bus line on the voltage of liquid crystal capacity in the multi-picture element driving depends on the voltage of the CS bus line at the time when the TFT element is turned OFF, i.e. at the time T2 of FIG. 7. Therefore, in timing when the gate of a TFT is turned OFF, when the potentials of the subsidiary capacity wires 24a differ from each other, how much the oscillation voltage of the CS bus line influences on the voltage of liquid crystal capacity differs, with a result that voltages applied to liquid crystal layers differ, and accordingly uneven luminance appearing in a lateral streak occurs.